Present developments in the field of lighting aim to replace common lighting devices, such as incandescent or halogen lamps, by retrofit lamps using light emitting diodes (LEDs). Such LED retrofit lamps exhibit reduced power consumption at a comparable luminous flux and an increased lifetime and are thus employed to increase the efficiency of lighting applications and to conserve electrical energy.
While the reduced power consumption of LEDs is beneficial to conserve energy, problems arise from the fact that the reduced power consumption results in an accordingly reduced operating voltage and current. For example, when an LED retrofit lamp is operated with a conventional power supply, such as an electronic transformer as used in halogen lighting systems, the transformer may have a minimum load requirement, which typically cannot be met by a LED retrofit lamp because of its much lower power consumption. Below said minimum load level, the operation of the electronic transformer may be unstable or result in no current being supplied to the lamp.
Although it is possible to adapt the LED lamp and increase the power consumption e.g. by increasing the number of LEDs used or by adding one or more resistors to the circuitry of the lamp, an increase of the power consumption certainly is prejudicial to the efficiency of the device and thus to the present efforts to conserve energy.
Document WO 2011/033415 of the present applicant provides a solution to the above problem. The device allows operating LEDs with a power supply having a minimum load requirement, such as an electronic transformer.
The document discloses an illumination device having a three-stage setup and a low-power light source, e.g. a light emitting diode. The disclosed device further comprises a power input stage using a boost converter, which boost converter is configured to draw current pulses from the power supply. During the pulses, the current level is high enough to meet the minimum load requirement of the transformer, so that electrical energy can be transferred to the illumination device. The power input stage is switched between a current generating mode and an OFF mode to set the power transferred to the lamp.
While the disclosed illumination device advantageously allows operating an LED light source with a power supply having a minimum load requirement, the present inventors recognized that the electrical efficiency may not be optimal under all operating conditions.
Therefore, it is an object of the present invention to provide an enhanced LED retrofit driver circuit on the basis of the disclosed setup, providing increased efficiency under a multitude of operating conditions.